B737-700 FLT CREW DURING DEP FROM DAL ON THE JOE POOL DEP HAS A HDG/COURSE DEV. DEP CTL VECTORS THE ACFT TO CORRECT COURSE.

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FLT DEPARTED DAL ON RWY 13R. CLBED RWY HDG TO 6000 FT; TOLD TO MAINTAIN 6000 FT THEN R HDG 180 DEGS WITH CLB TO 7000 FT; THEN R TO HDG 210 DEGS WITH CLB TO 10000. TOLD TO PROCEED DIRECT TO JASPA; CLB TO 17000 FT RESUME DEP. ADJUSTED HDG TO JASPA; THEN SELECTED LNAV AND NOTICED JASPA WAS COLORED MAGENTA ON B737-700 INBOARD DISPLAY UNIT. THEN RECEIVED CLRNC TO FL230. APPROX 2 TO 3 MINUTES LATER; ZFW ASKED IF WE WERE ON DEP ROUTING; THEN ISSUED AN IMMEDIATE L TURN TO 140 DEGS. AT THIS POINT; I NOTICED LNAV WAS NO LONGER ENGAGED; JASPA WAS COLORED WHITE 3 TO 4 MILES IN OUR 6 O'CLOCK. ACFT WAS FLYING LAST HDG PRIOR TO JASPA; BUT ACFT HAD NOT TURNED S ON PLANNED ROUTING. ZFW VECTORED ANOTHER ACFT TO MAINTAIN SEPARATION. PRIOR TO RE-ESTABLISHMENT ON DEP ROUTING; ZFW CLRED US DIRECT CENTEX AND CLRED US TO NEW FREQ. LNAV WORKED CORRECTLY REMAINDER OF DAY. UPON REFLECTION OF THE EVENT; IT'S CLEAR LNAV DISENGAGED AT SOME POINT; UNNOTICED. IT WAS PROBABLY OPERATOR ERROR; MOST LIKELY INADVERTENT; PERHAPS A KNUCKLE DESELECTED LNAV WHILE ADJUSTING ALT ON MCP?SUPPLEMENTAL INFO FROM ACN 693917: MY FIRST TIME TO FLY WITH THIS FO; FIRST LEG OF 4 DAY TRIP. DEP CTL TOLD US TO TAKE A HDG AND THEN GO RNAV DIRECT TO JASPA INTXN. FO SELECTED RNAV DIRECT JASPA AND CONTINUED CLB OUT OF 10000 FT. I WAS TALKING TO JUMPSEAT PAX. CTR/DEP CALLED US AND ASKED; 'HAVE YOU JOINED YOUR DEP YET?' I LOOKED AT MOVING MAP AND WE HAD PASSED JASPA AND MISSED JOINING OUR JOE POOL DEP. I RESPONDED TO CTR; 'WE ARE CORRECTING TO COURSE.' THE FO AND I WERE AT A LOSS TO FIGURE OUT WHY THE AUTOPLT CONTINUED PAST JASPA. I WAS DISTRACTED BY THE JUMPSEATER CONVERSATION AND DID NOT SEE THE DEVIATION.